Human immunodeficiency virus type 1 (HIV-1) is a member of the lentivirus subfamily of retroviruses. Like all retroviruses, lentiviruses encode the gag, pol, and env genes. However, lentiviruses also contain several accessory genes. The accessory gene vpu, which is unique to HIV-1, encodes viral protein U (Vpu) (44). Vpu is a 16-kDa type I integral membrane phosphoprotein that can form oligomeric structures in vitro and in vivo (32, 43). Indirect immunofluorescence indicates that Vpu localizes predominantly to the Golgi complex (29), but some Vpu is also present in association with the plasma membrane (17). The protein contains a hydrophobic N-terminal domain, which serves as the membrane anchor, and a C-terminal hydrophilic cytoplasmic domain (32).
Vpu plays two roles in HIV-1 replication. First, Vpu promotes the specific degradation of the HIV-1 receptor, CD4, in cell-free systems (8) and in vivo (41, 47). Degradation of CD4 enhances the transport and subsequent processing of the HIV-1 envelope glycoprotein by releasing it from complexes with CD4 that trap both proteins in the ER (28). Direct interaction of Vpu with the cytoplasmic domain of CD4 is required, but not sufficient, for CD4 degradation (4). Mutational analysis indicates that the hydrophilic cytoplasmic domain of Vpu is required for Vpu-mediated CD4 degradation (40). The second function of Vpu is the enhancement of virus particle release (19, 29, 43, 45, 48). The effect of Vpu on virus particle release appears to be mediated from a post-ER compartment (41). Whereas the cytoplasmic domain of Vpu is important for the degradation of CD4, the transmembrane domain of Vpu is sufficient for partial enhancement of virus release (40). Thus, based on both differential intracellular site of action and genetic criteria the bipartite roles of Vpu are mechanistically distinct. The HIV-1 Gag protein is sufficient for immature virus capsid formation, and those capsids are fully competent for Vpu-mediated enhancement of release, indicating that an eventual target of Vpu during particle release is intrinsic to Gag (30).
The identification of host cell proteins that function in HIV replication has provided crucial insight into the intricacies of the biology of HIV-1. The identification of CD4 as the principal virus receptor on T-cells has provided a basic paradigm for virus entry (10). Chemokine receptors are proteins involved in chemotaxis of immune system cells and have been co-opted by HIV-1 to allow entry into host cells in conjunction with CD4 (2, 13). Urokinase-type plasminogen activator, a proteinase involved in tissue invasion by macrophages, binds to and cleaves the HIV-1 envelope glycoprotein gp120 and enhances the infectivity of HIV-1 in macrophages (24). Cyclophilins are proteins that bind to the immunosuppressive drug cyclosporin A (CsA) and are members of the immunophilin superfamily, which includes members that facilitate protein folding (18). Cyclophilins A, B, and C interact with HIV-1 Gag, and cyclophilin A is incorporated into virions (15, 31, 46). The incorporation of cyclophilin A into virus particles is required for an early step in replication between membrane fusion and reverse transcription (5). Furin, a subtilisin-like endoprotease, mediates the cleavage of the HIV-1 envelope glycoprotein precursor gp160 to gp120 and gp41, a process required for virus infectivity (22).